Tough question. I was a teenager and an emerging film geek during the 80's and saw a lot of movies. A lot of forgettable crap, a lot of wonderful foreign films, some movies girlfriends dragged me to, and just about every giant blockbuster released during the era. Looking back, the ONE film that defines the decade for me is James Cameron's Aliens. I'm not arguing it's the best film of the era, only that it stands out in my memory as the singular movie going experience of the time. To this day, I've never heard a more rousing audience reaction than when Ripley shouts at the Alien Queen "Get away from her, you BITCH!"